The aim of the proposed research is to develop chemical and enzymatic approaches to study nucleic acid structure and nucleic acid- protein interactions. The present studies focus on the interactions of restriction and modification enzymes with the specific DNA sequences. To study the mode of action, specificity and requirement of several restriction and modification enzymes, a DNA (of 22 nucleotides in length) of well defined sequence will be chemically synthesized. The synthetic DNA will carry selected nucleotide sequences recognized by restriction and modification enzymes. For studies of enzyme-DNA complexes, a DNA pseudosubstrate will be synthesized in which the phosphodiester bond at the site of action will be replaced by its methylene analogue. Such a complex will then be crystallized and used for x-ray diffraction analysis. Chemical methods will be developed for the synthesis of short DNA's (8 to 10 nucleotide long duplexes) carrying chemically- or photo-reactive groups. Such modified substrates will then be used to affinity label the active site of the enzyme. The base specificity of endonucleases such as T4 endonuclease IV, streptodornase, E. coli K-12 endonucleases A1, A2, A3, etc., will be studied using synthetic DNA's.